Now I haven't done this myself as I don't blog by email but I did notice it while I was delving for other stuff so...

I think you need to edit conf/_advanced.php

Find the section that looks like this or similar

// ** Configuration for htsrv/getmail.php **
// (skip this if you don't intend to blog via email)
# mailserver settings
$mailserver_url = 'mail.example.com';
$mailserver_login = 'login@example.com';
$mailserver_pass = 'password';
$mailserver_port = 110;
# by default posts will have this category
$default_category = 1;
# subject prefix
$subjectprefix = 'blog:';
# body terminator string (starting from this string, everything will be ignored, including this string)
$bodyterminator = "___";
# set this to 1 to run in test mode
$thisisforfunonly = 0;

It all seems pretty self explanatory

:D